1. Field of the Invention
The present invention relates to a free space optics communication apparatus which uses a light signal propagating through the air to perform communication from a single point to a plurality of points at remote locations.
2. Description of the Related Art
Free space optics communication apparatuses for so-called point-to-multipoint communication have been proposed which use light signals propagating through the air to perform communication from a single point to a plurality of points at remote locations simultaneously. Some of the free space optics communication apparatuses are of a type which realizes the communication by providing a predetermined number of movable mirrors disposed in luminous flux emitted from a single light source and setting different directions of the movable mirrors.
FIG. 15 shows an example of the apparatus configured as described above. In FIG. 15, reference numeral 70 shows a free space optics communication apparatus installed on the one point side of the point-to-multipoint communication. Reference numerals 71a, 71b, and 71c show other spatial light apparatuses on the multipoint side installed at a plurality of points at remote locations, with which the apparatus 70 is to communicate.
A light signal emitted from a light source 701 is converted by an optical system 702 into a light beam 703 which has a substantially parallel shape with slight divergence. The light beam 703 is divided and reflected by three movable mirrors 704a, 704b, and 704c disposed in the luminous flux of the beam to travel toward the other apparatuses 71a, 71b, and 71c separately. In this manner, the signal is transmitted to the respective other apparatuses.
Each of the movable mirrors 704a, 704b, and 704c is configured to allow free setting of its direction. The directions of the movable mirrors 704a, 704b and 704c are set when the apparatus is installed such that reflected light beams can be transmitted toward the respective other apparatuses 71a, 71b, and 71c. 
Such a configuration can realize the free space optics communication apparatus for point-to-multipoint communication which can communicate with a plurality of other apparatuses through the light beam emitted from the single light source 701.
It is rare that the plurality of other apparatuses is installed at equal distances from the free space optics communication apparatus. In most cases, the respective other apparatuses are installed at different distances.
Conventionally, however, the plurality of movable mirrors generally have the same sizes. This leads to a large difference in communication quality between a case where the apparatus communicates with a different apparatus installed at a long distance and a case where the apparatus communicates with the other apparatus installed at a short distance.
On the other hand, when different levels of communication quality are required by the plurality of other apparatuses, it is difficult for the conventional apparatus to flexibly set the communication quality to satisfy the requirements.
In the aforementioned related art, the number of the movable mirrors in the free space optics communication apparatus is determined in the manufacturing stage of the apparatus. However, in an environment in which the apparatus is actually used, the number of movable mirrors in the apparatus is not always consistent with the number of other apparatuses to be communicated with, and is rather inconsistent in many cases.
When the actual number of the other apparatuses is smaller than the number of the movable mirrors provided in the apparatus, a certain number of movable mirrors are not used corresponding to the difference between them. Light from the light source is always made incident on the movable mirrors which are not used, and such light incident on those movable mirrors unnecessary. For example, when an apparatus having four movable mirrors is used for communication with two other apparatuses installed at two locations, only half of the power of the light source is effectively used.
On the other hand, when the number of other apparatuses is larger than the number of the movable mirrors provided in the apparatus only by one, for example, an additional free space optics communication apparatus needs to be installed. In this manner, the conventional free space optics communication apparatus has poor flexibility for use in actual point-to-multipoint communication to result in the inability to effectively utilize the performance which the free space optics communication apparatus has originally.